Throughout the text, the following terminology is adopted:                nanoparticle: any particle of whatever shape having at least a width and thickness which are both less than 100 nm, typically of between 1 nm and 20 nm;        metal: including at least one metal atom, in particular which can be chosen from: gold, silver, platinum, rhodium, iron, cobalt, copper, nickel, zinc, tin, titanium, manganese, chromium, vanadium, indium, ruthenium, palladium, molybdenum, niobium, zirconium, tantalum, aluminum, gallium, tungsten, rhenium, osmium, iridium;        non-oxidized metal nanocrystals: nanoparticles made up of at least one pure metal compound in the non-oxidized crystalline state, each nanoparticle having the metal structure, that is to say being formed from atoms of metal(s) bonded to one another as in a bulk metal;        oxidized metal nanocrystals: nanoparticles made up of at least one pure metal compound in the crystalline state, but these nanoparticles having been subjected to an at least partial oxidation after their formation, from an initial state in the form of non-oxidized metal nanocrystals;        metal nanocrystals: non-oxidized metal nanocrystals or oxidized metal nanocrystals;        organometallic precursor: any molecule or coordination compound containing at least one organic grouping bonded to at least one metal atom by a carbon atom or a hetero atom, excluding oxygen (chosen in particular from N, P, As, Si, S, Se, Te), of this organic grouping;        carbon chain: any aliphatic chain, saturated or unsaturated, straight or branched, substituted or unsubstituted, which may include hetero atoms;        solvent medium: any composition in which water and dioxygen can be present only in traces and which is capable of forming a liquid solution when brought into contact with at least one compound such as an organometallic precursor; it can be in the initially liquid state or, on the other hand, can pass into the liquid state only after contact with the compound(s) to be solubilized; it can be simple, that is to say formed by a single compound, or on the other hand complex and comprise several compounds; in particular, it can comprise not only one or more compound(s) which act as the solvent agent, but also any other compound which is not consumed by the formation reaction of metal nanocrystals—in particular in a reduction reaction—is substantially neutral with respect to the dissolution of the organometallic precursor(s), and possibly plays a role in the formation reaction of metal nanocrystals—in particular in a reduction reaction;        colloidal solution: any clear liquid composition of solid nanoparticles dispersed in a liquid; a liquid colloidal solution has several but not all of the properties of a true liquid solution, the nanoparticles remaining in the solid state; colloidal suspension or dispersion is also sometimes referred to;        water-compatible composition of nanoparticles: any composition of nanoparticles which can be dispersed at least in an aqueous medium, in particular any composition which can form a colloidal solution (liquid dispersion) in an aqueous medium;        organic-compatible composition of nanoparticles: any composition of nanoparticles which can be dispersed in at least an organic—in particular non-aqueous—protic or aprotic medium, in particular any composition which can form a colloidal solution (liquid dispersion) with at least such an organic—in particular non-aqueous—protic or aprotic liquid medium;        coordination grouping: any chemical grouping which can form a covalent, dative, hydrogen or electrostatic bond with metal atoms, metal ions, oxygen and metal oxides.        
FR 2678855 describes a method for the preparation of a dispersion of metal particles consisting of dissolving an organometallic precursor and a cellulose matrix in a common organic solvent and allowing a reducing agent to act on the solution in order to decompose the precursor into metal particles. This method is satisfactory and enables production of compositions of metal particles which are organic-compatible but which on the other hand are not water-compatible, that is to say cannot be dispersed in an aqueous solvent. It thus does not enable a water-compatible composition of metal nanocrystals to be obtained.
In this respect it is to be noted that the presence of an uncontrolled amount of water is strictly incompatible with a controlled reaction in the presence of organometallic(s). In fact, in the organometallic technical field, water is systematically considered to be detrimental, and indeed a hazard. More particularly, in the case of a reduction reaction starting from an organometallic precursor, it is considered that the presence of an uncontrolled amount of water in the medium would necessarily have the consequence at the very least of considerably disturbing and even preventing the functioning of the reaction. In fact, it is known that any uncontrolled presence of water unavoidably leads to the formation of metal hydroxides (destructive and exothermic decompositions of the Zerewitinoff type) and is destructive and detrimental in the context of the preparation and use of organometallic compounds. Needless to say, reactions in the presence of organometallics are most often carried out in the presence of a water trap in order to work in a dry atmosphere.
It would therefore be useful to enable such compositions of metal nanocrystals which are water-compatible, and more particularly both organic-compatible and water-compatible, that is to say which can be dispersed both, and as required, in aprotic—in particular organic non-aqueous—media and in protic media—in particular water and aqueous media—to be obtained. In particular it is important to obtain such water-compatible compositions to enable them to be used in numerous applications, in particular in physiological media, for therapeutic use or for medical imaging, and in all applications for which the aim is to avoid the use of organic solvents which are toxic and/or polluting volatile organic compounds (VOC), the use of which must be limited and even suppressed taking account of environmental awareness regulations.
In addition, the preparation of water-compatible non-oxidized metal nanocrystals in a first stage and dispersion thereof in an aqueous medium in a second stage could enable, in the case of certain oxidizable metals, nanocrystals of very small dimensions to be obtained in the at least partially oxidized state.
It has already been proposed to prepare colloidal solutions of metal particles in an aqueous medium by means of a reducing agent dissolved in the medium (ascorbate, citrate . . . ) at the reflux temperature in the presence or absence of a stabilizer. Such a method does not enable an organic-compatible composition to be obtained, is accompanied by the formation of contaminating secondary products, and does not enable nanometric particles having at least one dimension smaller than 5 nm to be obtained.
Various methods have also already been proposed to enable compositions of metal nanocrystals which are initially not water-compatible to be rendered water-compatible.
A first approach could consist of exchanging the hydrophobic ligands for ligands which are analogous but have hydrophilic groups, such as polymers derived from PEG (thiol-PEG, amino-PEG, carboxy-PEG). However, this approach would necessitate a relatively complex second stage, the yield of which is not very good. In addition, it would not result in nanocrystals doped exclusively with hydrophilic ligands, the exchange reaction never being total.
A second approach consists of incorporating into the composition obtained amphiphilic ligands which are capable of interacting with the hydrophobic ligands resulting from the preparation of the nanoparticles, without replacing these hydrophobic ligands, forming bilayer structures around the nanoparticles. The compositions obtained with this approach may have a certain toxicity (due to release of amphiphilic compounds) and a poorly controlled stability, which is a disadvantage in particular in biological and therapeutic applications.
In certain very specific cases, another approach can consist of choosing a ligand having at one of the ends of the aliphatic alkyl chain a grouping which subsequently enables chemical reactions for grafting of a hydrophilic grouping to be carried out.
But, there again, an additional stage is necessary, and this approach is only possible in very particular cases which are of little use in practice.
In addition, these various approaches also most often have the disadvantage that the compositions of nanoparticles which have been modified to be water-compatible subsequently are no longer organic-compatible under satisfactory conditions.
The object of the invention is thus to propose a method for preparing a composition of metal nanocrystals which, on the one hand, is both organic-compatible and water-compatible, which to date was considered to be absolutely impossible, and, on the other hand, in which the nanocrystals have at least one dimension—in particular an average dimension—smaller than 5 nm.
More particularly, the object of the invention is to propose a method which enables a composition of metal nanocrystals in the form of a colloidal solution, regardless of the solvent medium, organic or aqueous, and of which the properties remain unchanged in an aqueous medium, to be obtained.